1. Field of the Invention
The present invention relates to a metal-graphite brush containing graphite, a metal, and a lubricant, which brush is employed in miniature motors.
2. Background Art
Conventionally, metal-graphite brushes (carbon brushes) have been employed in miniature motors (see Japanese Patent Application Laid-Open (kokai) No. H07-59299). FIG. 13A is a vertical cross-sectional view of essential portions of such a common miniature motor; and FIG. 13B is a cross-sectional view of the motor shown in FIG. 13A, as taken along line A-A. As shown in FIGS. 13A and 13B, a permanent magnet is fixed to the inner circumferential surface of a bottomed hollow tubular casing made of a ferromagnetic metallic material. A rotor is formed of an armature facing the permanent magnet and a commutator so that the rotor can be provided in the casing. A casing cap is fitted into the opening of the casing. A carbon brush is provided on a tip end portion of a brush arm made of an electrically conductive material so that the carbon brush is slidably engaged with the commutator. The brush arm and an input terminal electrically connected thereto are provided in the casing cap. The shaft of the rotor is rotatably supported by bearings which are respectively provided in the casing and the casing cap.
A conventional carbon brush (metal-graphite brush) employed in such a miniature motor contains graphite, a metal (generally copper), and a lubricant (generally molybdenum disulfide), and the carbon brush exhibits long service life and excellent stability over time. However, in recent years, further improvement has been required in characteristics in high temperature and humidity environments of a carbon brush employed in, for example, automotive electrical components. In a high-temperature and high-humidity environment, a carbon brush would otherwise be impaired through oxidation, resulting in deterioration of motor characteristics. Therefore, demand has arisen for prevention of impairment through oxidation of a carbon brush for suppressing deterioration of motor characteristics, as well as improvement of the service life of the carbon brush.
Japanese Patent Application Laid-Open (kokai) No. 2001-327127 or 2003-272795 discloses a copper-carbon brush containing bismuth. However, the copper-carbon brush disclosed in Japanese Patent Application Laid-Open (kokai) No. 2001-327127, which contains a small amount of bismuth (about 0.005 to about 0.05 wt. %), was found not to satisfy recent requirements for characteristics in high temperature and humidity environments (see FIG. 3 shown hereinbelow). Meanwhile, Japanese Patent Application Laid-Open (kokai) No. 2003-272795 disclosed only addition of bismuth to a copper-carbon brush, and does not specifically disclose, for example, the bismuth content of the brush.